1. Field of the Invention
This invention relates to a combined cycle engine system that combines a turbojet or other booster engine and a Dual Mode Ramjet (DMRJ) to permit efficient operation from takeoff to hypersonic speeds and more particularly from Mach 0 Sea Level to Mach 5+ at high altitude.
2. Description of the Related Art
A conventional DMRJ cannot produce thrust to accelerate itself to supersonic speeds. The DMRJ must be boosted by some other propulsive element, such as a Turbojet Engine (TJ). When the booster engine is a turbojet (turbine) engine, the combined cycle engine is referred to as a Turbine Based Combined Cycle or TBCC engine. If the booster is a rocket engine, it is referred to as a Rocket Based Combined Cycle or RBCC engine. In prior art reusable hypersonic vehicle concepts, the turbine engines of a TBCC engine are expected to produce all of the thrust at subsonic and low supersonic speeds. At some higher speed, the DMRJ is turned on to produce the required thrust and the TJ is turned off at nearly the same speed and taken out of the flow. The thrust of the TJ engine as the sole propelling means during the bulk of the acceleration places a great demand on the TJ technologies. Prior art TBCC engines have little or no DMRJ thrust contribution at speeds below Mach 3-4. The highest thrust requirement for these vehicles occurs during acceleration from subsonic to supersonic speeds. This so called “transonic” speed has the greatest drag to overcome. From the foregoing, it is seen that the greater the thrust contribution from the DMRJ during acceleration the less demands are placed on the turbine engine.
Current turbine engine technology is suitable for speeds up to Mach 2.5. Above this speed, the air temperature becomes too high to permit high compressor pressure ratio without exceeding the turbine entrance temperature limits. This results in a reduction in engine airflow and thrust. A Lockheed SR-71 high-speed, high-altitude, reconnaissance aircraft was able to fly at about Mach 3.25 by bypassing some of the air around the final compressor stages in the Pratt & Whitney J-58 engine. This unloaded the compressor, reducing the combustion heat addition required upstream of the turbine. Such an engine cycle is referred to as a Turbo-Ramjet since most of the high Mach thrust is produced in an afterburner downstream of the turbine. This engine cycle is not a preferred cycle for a TBCC system which would need a third duct for DMRJ operation. Since the DMRJ duct must operate at speeds beyond what the TJ can stand, the DMRJ air flow must bypass the TJ completely. One important issue with the prior art TBCC engines is the requirement for the Turbine engine to operate to Mach 4 or higher. This places a large technical hurtle to develop a pure Turbojet that can operate with high thrust at Mach 4 or higher. The thrust a TJ can produce as a function of Mach number is dependent on the technologies applied. For Mach 4 operation at high thrust, advanced high strength high temperature materials are needed that are not currently available.
When not operating, the DMRJ flowpath increases vehicle drag if air flows through the duct or around it. At speeds below typically Mach 5, the TBCC nozzle is over-expanded (too large) which reduces the net thrust. Increasing the size of the turbine to produce sufficient thrust to overcome the vehicle and the non-operating DMRJ engine drag has severe mission consequences due to greater vehicle empty weight and reduced available fuel volume. United States patent application publication number US 2006/0107648 A1 by Bulman et al. discloses a TBCC having an integrated inlet that manages the flow of air to both the TJ and DMRJ elements. The US 2006/0107648 A1 patent application is incorporated by reference in its entirety herein.
It is known that the thrust in a DMRJ at low speeds is limited due to low ram pressure and premature thermal choking of the combustor. We address each of the limiting factors on low Mach TBCC thrust:
Subsonic to Low Supersonic Thrust—As a ram compression cycle, the DMRJ has little thrust potential at low speeds. For typical TBCC powered hypersonic vehicles the drag at transonic speeds (Mach 0.8-1.3) is usually more than the turbine engine can produce. Additional thrust is needed but just installing a larger turbojet engine is unattractive in a weight and volume sensitive hypersonic vehicle.
Low Supersonic to Mach 4 Thrust—Prior art DMRJs have a Scramjet diverging combustor and an isolator to allow operation with a thermal throat. These engines are usually not capable of producing useful thrust much below about Mach 4. A first factor is a low inlet/isolator pressure rise capability at low speeds. A second factor is that at low speeds and typical combustor area ratios, the pressure rise for a given fuel equivalence ratio increases at low supersonic speeds. FIG. 1 shows the temperature rise to thermally choke a DMRJ as a function of speed and combustor Area Ratio (AR). A typical prior art DMRJ has a small AR, on the order of 2 (Reference line 10), suitable for the higher speeds. The engine thrust is directly related to the temperature rise. If too much heat is added with a low AR combustor, the combustor pressure will exceed the inlet capability and the inlet will unstart. This miss-match between available pressure and combustor back pressure prevents practical thrust from a prior art DMRJ. Below Mach 4, a high area ratio combustor improves this situation (Reference line 12), but is inefficient for high speed operation since it increases the wetted area exposed to the high speed, high enthalpy, flow—increasing engine weight and heat load while reducing high speed thrust.
TBCC Combined Thrust—Typical prior art hypersonic cruise vehicles have as large a nozzle exit area as practical since at cruise speed the exhaust is under-expanded and thrust and Isp increase with larger nozzles. At low speeds, the exhaust is then over-expanded and thrust and Isp are lower than they would be with a smaller nozzle. One solution to this problem, creating a larger volume of gas to help fill and pressurize the nozzle, is disclosed in U.S. Pat. No. 6,568,171 to Bulman. U.S. Pat. No. 6,568,171 is incorporated by reference in its entirety herein.